Alessandro La Torre

Encapsulation of single-molecule magnets in carbon nanotubes

Next-generation electronic, photonic or spintronic devices will be based on nanoscale functional units, such as quantum dots, isolated spin centres or single-molecule magnets. The key challenge is the coupling of the nanoscale units to the macroscopic world, which is essential for read and write purposes. Carbon nanotubes with one macroscopic and two nanoscopic dimensions provide an excellent means to achieve this coupling. Although the dimensions of nanotube internal cavities are suitable for hosting a wide range of different molecules, to our knowledge, no examples of molecular magnets inserted in nanotubes have been reported to date. Here we report the successful encapsulation of single-molecule magnets in carbon nanotubes, yielding a new type of hybrid nanostructure that combines all the key single-molecule magnet properties of the guest molecules with the functional properties of the host nanotube. The findings may pave the way to the construction of spintronic or ultrahigh-density magnetic data storage devices.

Assembly, Growth, and Catalytic Activity of Gold Nanoparticles in Hollow Carbon Nanofibers

Graphitized carbon nanofibers (GNFs) act as efficient templates for the growth of gold nanoparticles (AuNPs) adsorbed on the interior (and exterior) of the tubular nanostructures. Encapsulated AuNPs are stabilized by interactions with the step-edges of the individual graphitic nanocones, of which GNFs are composed, and their size is limited to approximately 6 nm, while AuNPs adsorbed on the atomically flat graphitic surfaces of the GNF exterior continue their growth to 13 nm and beyond under the same heat treatment conditions. The corrugated structure of the GNF interior imposes a significant barrier for the migration of AuNPs, so that their growth mechanism is restricted to Ostwald ripening. Conversely, nanoparticles adsorbed on smooth GNF exterior surfaces are more likely to migrate and coalesce into larger nanoparticles, as revealed by in situ transmission electron microscopy imaging. The presence of alkyl thiol surfactant within the GNF channels changes the dynamics of the AuNP transformations, as surfactant molecules adsorbed on the surface of the AuNPs diminished the stabilization effect of the step-edges, thus allowing nanoparticles to grow until their diameters reach the internal diameter of the host nanofiber. Nanoparticles thermally evolved within the GNF channel exhibit alignment, perpendicular to the GNF axis due to interactions with the step-edges and parallel to the axis because of graphitic facets of the nanocones. Despite their small size, AuNPs in GNF possess high stability and remain unchanged at temperatures up to 300 °C in ambient atmosphere. Nanoparticles immobilized at the step-edges within GNF are shown to act as effective catalysts promoting the transformation of dimethylphenylsilane to bis(dimethylphenyl)disiloxane with a greater than 10-fold enhancement of selectivity as compared to free-standing or surface-adsorbed nanoparticles.

Transport and encapsulation of gold nanoparticles in carbon nanotubes.

Nanoparticles confined in small volumes exhibit functional properties different from that of the bulk material. Furthermore, the smaller the volume available then the greater the effects of confinement are observed to be. Metallic nanoparticles encapsulated within carbon nanotubes have been proposed for many applications ranging from catalysis to quantum storage devices. In this study we examine encapsulation of discrete gold nanoparticles (AuNP) within multi-wall carbon nanotubes (MWNT), with internal diameter less than 10 nm. During the encapsulation process AuNP undergo Ostwald ripening allowing them to reach a diameter that precisely matches the internal diameter of MWNT (snug fit). The use of supercritical CO2 as a processing medium enables efficient transport and irreversible encapsulation of AuNP into narrow nanotubes. Once inside MWNT, the nanoparticles are unable to grow further and retain their spheroidal shape. This dynamic behaviour observed for AuNP differs significantly from the behaviour of molecular guest-species under similar conditions.

Controlling the Regioselectivity of the Hydrosilylation Reaction in Carbon Nanoreactors

Hollow graphitized carbon nanofibres (GNF) are employed as nanoscale reaction vessels for the hydrosilylation of alkynes. The effects of confinement in GNF on the regioselectivity of addition to triple carbon-carbon bonds are explored. A systematic comparison of the catalytic activities of Rh and RhPt nanoparticles embedded in a nanoreactor with free-standing and surface-adsorbed nanoparticles reveals key mechanisms governing the regioselectivity. Directions of reactions inside GNF are largely controlled by the non-covalent interactions between reactant molecules and the nanofibre channel. The specific π-π interactions increase the local concentration of the aromatic reactant and thus promote the formation of the E isomer of the β-addition product. In contrast, the presence of aromatic groups on both reactants (silane and alkyne) reverses the effect of confinement and favours the formation of the Z isomer due to enhanced interactions between aromatic groups in the cis-orientation with the internal graphitic step-edges of GNF. The importance of π-π interactions is confirmed by studying transformations of aliphatic reactants that show no measurable changes in regioselectivity upon confinement in carbon nanoreactors.

Interactions of Gold Nanoparticles with the Interior of Hollow Graphitized Carbon Nanofibers

Interactions of free-standing gold nanoparticles and hollow graphitized nanofibers in colloidal suspension are investigated, revealing the first example of the controlled arrangement of nanoparticles inside nano-containers, as directed by their internal structure. The ordering is highly effective for small gold nanoparticles whose sizes are commensurate with the height of graphitic step-edges in the graphitized carbon nanofibers and is less effective for larger gold nanoparticles. Studies aimed at understanding the role of the organic-solvent surface tension, employed for the filling experiments, demonstrate that gold nanoparticles become preferentially anchored into the hollow graphitized carbon nanofibers under a mixture of pentane/CO(2) in supercritical conditions. It is shown that a three-step cleaning procedure enables effective removal of gold nanoparticles adsorbed on the exterior surface of graphitized carbon nanofibers, while ordered arrays of encapsulated nanoparticles are retained.

Assembly and Magnetic Bistability of Mn3O4 Nanoparticles Encapsulated in Hollow Carbon Nanofibers

Next-generation spintronic and data storage devices will be based on nanoscale functional materials such as magnetic nanoparticles. One particular challenge for harnessing the magnetic bistability, quantum tunnelling of magnetization, and quantum coherence of nanometer-sized magnetic objects is their coupling to the macroscopic world. Hollow carbon nanostructures with one macroscopic and two nanoscopic dimensions provide excellent materials to achieve this coupling, through the encapsulation and confinement of magnetic species. The insertion of magnetic nanoparticles into carbon nanostructures has been achieved mainly through the sublimation of a metal precursor, or the capillarity filling of a molten metal salt followed by pyrolysis of the encapsulated material. The main drawback of these approaches is a lack of control over the composition, size, and morphology of the nanoparticles formed inside the nanotubes. Since the functional properties of nanometer-sized magnetic objects are strongly dependent on these parameters, precise methods for encapsulation are required. The insertion of preformed nanoparticles with well-defined magnetic properties into carbon nanostructures, under conditions where their structures and properties are fully retained, could offer a powerful route for the development of novel architectures for spintronic devices. To date, the encapsulation of preformed nanoparticles has been reported only for nonmagnetic metals. Even though the size, shape, and composition of preformed nanoparticles can be effectively controlled by various preparation methods, the control of nanoparticle assemblies and their associated properties, combined with their confinement within carbon nanostructures, still remains a challenge. Here, we report the first example of the encapsulation of preformed, non-equiaxed, magnetic nanoparticles (NPs) within hollow carbon nanofibers (NFs), demonstrating the importance of the host-container internal structure on the NP assemblies and hence their collective magnetic properties. Two different types of hollow carbon nanofibers with different internal surface morphologies were employed to investigate the effects of confinement on the NP assembly and magnetic properties of the resultant hybrid nanostructures. The first type, a herringbone carbon nanofiber (CNF), comprised individual graphene layers tilted with respect to the main axis of the nanofiber, forming a uniform infinite stack (Figure 1A–C). The second type, a graphitized carbon

Biotechnological promises of Fe-filled CNTs for cell shepherding and magnetic fluid hyperthermia applications.

Fe-filled carbon nanotubes (Fe@CNTs) recently emerged as an effective class of hybrid nanoparticles for biotechnological applications, such as magnetic cell sorting and magnetic fluid hyperthermia. Aiming at studying the effects of both the Fe loading and the magnetocrystalline characteristics in these applications, we describe herein the preparation of Fe@CNTs containing different Fe phases that, upon functionalization with the antibody Cetuximab (Ctxb), allow the targeting of cancer cells. Our experimental findings reveal that an optimal Ctxb/Fe weight ratio of 1.2 is needed for efficient magnetic cell shepherding, whereas enhanced MFH-induced mortality (70 vs. 15%) can be reached with hybrids enriched in the coercive Fe(3)C phase. These results suggest that a synergistic effect between the Ab loading and the Fe distribution in each nanotube exists, for which the maximum shepherding and hyperthermia effects are observed when higher densities of Fe@CNTs featuring the more coercive phase are interfaced with the cells.

Dynamics of Gold Nanoparticles on Carbon Nanostructures Driven by van der Waals and Electrostatic Interactions

Transmission electron microscopy studies on the assembly and growth of gold nanoparticles on carbon nanotubes supported on few-layer graphene and amorphous carbon reveal a competition between van der Waals forces and electrostatic interactions, enabling controlled positioning and sizing of adsorbed nanoparticles at the nanochannels formed between the carbon nanotube and the few-layer graph-ene surface.

Controlled oxidative cutting of carbon nanotubes catalysed by silver nanoparticles

A systematic comparison of different methods for the cutting of carbon nanotubes has shown that oxidation catalysed by silver nanoparticles is a superior method for the efficient procurement of short carbon nanotubes with minimal introduction of sidewall damage. Complementary examination by microscopic and spectroscopic approaches indicates that the mean length of multi-walled carbon nanotubes synthesised by chemical vapour deposition can be reduced by a factor of five or more while preserving sidewall structure. We have demonstrated the versatility of this procedure by application to an array of hollow carbon nanostructures possessing a diverse range of structural characteristics, such as length, diameter, internal and external structures, with the extent of cutting seemingly related to the initial degree of structural imperfection in the nanotube.

Electrical properties of atomic carbon chains measured by in-situ TEM

Carbon in the sp1 hybridization (carbyne) is able to form atomic chains that constitute the elementary one-dimensional phase of carbon [1]. Despite many efforts, the synthesis of carbon chains in appreciable quantities remains difficult and even the existence of bulk carbyne is subject of an ongoing controversy. However, the existence of individual carbon chains is undisputed since they have been observed in in-situ TEM studies. The present work allowed, for the first time, to measure the electrical properties of carbon chains by using a dedicated specimen stage for establishing electrical contacts in the TEM. Carbon chains are unusual conductors that may occur either as metallic cumulene with double bonds or as semiconducting polyyne with alternating single and triple bonds. Now, as it became possible to characterize them electrically, these two electronic configurations can be distinguished. A piezo-driven tip (Nanofactory), integrated into the specimen holder of a TEM, allowed to establish contacts to graphenic material and, by controlled retraction of the electrodes, to unravel chains of carbon atoms (fig. 1a). At the same time, the electrical properties could be measured [2, 3]. By recording current-voltage curves of individual carbon chains, both cumulene and polyyne were identified (fig. 1b). It was found experimentally and by quantum conductance calculations (fig. 1c) that transport through narrow resonant states makes the conductivity much lower than predicted in previous theoretical work. At high applied bias, however, a sudden rise in current occurs, showing the absence of conduction channels at lower energy and their presence at higher energy. When the 1D system is under strain, the chains exhibit a semiconducting behavior, corresponding to polyyne. Conversely, when the chain is unstrained, an ohmic behavior, corresponding to cumulene, is observed (fig. 1b) [4]. This confirms a recent theoretical prediction, namely that the Peierls distortion, which would stabilize polyyne, is suppressed by zero-point vibrations in an unstrained chain so that cumulene is the stable configuration. In the presence of strain, however, polyyne is favoured by the Peierls instability. Thus, a metal-insulator transition can be induced by adjusting the strain. Furthermore, it is shown that these atomic chains can act as rectifying diodes when they are in a non-symmetric contact configuration, i.e., between a carbon and a metal contact or between two carbon contacts of different type. Keywords: in-situ electron microscopy; carbon; carbyne; one-dimensional materials